Rolf Harris began a long-running relationship with an 18-year-old friend of his daughter after being flattered by her attentions, he told a court on Tuesday, but he vehemently denied a string of indecent assaults against girls and women spanning more than two decades.

"I felt I had betrayed everybody, my wife, my daughter, and her parents," the Australian-born entertainer, now 84, told Southwark crown court, saying the relationship with the teenager, 35 years his junior, left him "sickened by myself".

The woman, formerly a friend of Harris's daughter, Bindi, has told the court she was repeatedly groped by the artist and television star during the late 1970s, when she was 13. But beginning his defence case, Harris insisted that nothing happened until five years later.

Dropping his voice to a near murmur following a boisterous opening section of testimony in which he mimicked a didgeridoo and a wobble board and briefly sang one of his hits Jake the Peg, Harris said the teenager was staying at his family home in Bray, Berkshire, in the early 1980s when he brought her a cup of tea in bed. Without warning she kicked off the duvet and was half-naked, Harris said. "It seemed to me she was being very flirtatious with me. As you can imagine it was a very flattering feeling, this young lady to be suddenly having an interest in me."

The sexual contact began when the 18-year-old stayed again a few weeks later and appeared "very flirtatious, very coquettish", he told the court. Asked by his defence counsel, Sonia Woodley QC, if the teenager was willing, Harris replied: "Yes, she was. Definitely."

Harris, who told the court he was "a very touchy-feely type of person" who habitually hugs those he likes, described in sometimes graphic detail a series of subsequent encounters which lasted until the woman was in her late 20s. All of these, he said, left him deeply remorseful: "I knew it was wrong. Here I was, much older than her. A very young lady who was many years my junior. It was illicit, and a guilty feeling. As a married man it was just general guilt about the whole thing, and about her being Bindi's friend."

Such was his embarrassment, he said, that his initial statement to police in 2013 failed to mention any sexual contact before the woman was in her 20s as he could not bring himself to tell even his lawyers.

Harris described an encounter with the woman in the early 1990s, after their relationship had ended, when she was drunk and threatened to go to the media. He said: "It was like the sword of Damocles hanging over me. I was waiting each weekend for a newspaper article destroying me."

The woman has told the court the alleged assaults left her dependent on drink and prone to panic attacks. Harris said he felt partly culpable, which is why he wrote to the woman's father to beg forgiveness. "When I saw her drunken state and realised that she blamed me for all of it I took that on board and felt that maybe it was my fault, although I couldn't believe it." Harris was also asked about the family of the alleged victim, saying he had little to do with them. He said: "We had absolutely nothing in common. They were very straight up and down and did everything exactly right. We could never think of anything to say to them when we were with them. I think we were too bohemian and artistic for them."

Harris, speaking to a packed court including his wife of 56 years, Alwen, said he had another affair, this time with a woman in her mid-30s who briefly stayed rent-free with Harris and his wife when she was "down on her luck" in the mid-1990s. She began chauffeuring him to a pantomime in which he was performing and the relationship "gradually became physical", Harris said.

"I didn't feel good about it," he said, saying of his wife: "She was devastated by it, and rightly so."

Harris faces 12 charges of indecent assault, seven of which relate to his daughter's friend, with the court also hearing from a series of witnesses who allege similar attacks which did not result in charges, largely as they took place outside the UK before offences could be prosecuted here. The combined claims cover a period from the late 1960s to 1991.

Woodley led Harris through each of these, and he denied them all. Asked about an allegation that he groped her when she was eight at an appearance at a leisure centre in Portsmouth in 1969, Harris said he was then too famous to be at such modest events and was anyway in Australia at the time. Asked about an alleged assault on a girl of 11 in Australia after saying he wanted to give her a first tongue kiss, Harris replied: "First and foremost I wouldn't say that to anybody, and it didn't happen."

The opening section of testimony saw Harris take the court through a 60-year career that began when he arrived in London aged 22 to study art before getting a break on BBC children's television, appearing alongside a puppet called Fuzz.

In exchanges that caused occasional giggles Woodley took Harris at length through the invention of the wobble board, his trademark musical instrument, and his mastery of the didgeridoo, with the entertainer giving impressions of both. Asked about the genesis of one of his most famous songs, Jake the Peg, about a man with a third leg, Harris briefly sang the chorus to the court. At times, Woodley asked Harris to stick to the subject, saying at one point: "Can we just concentrate on how the wobble board came into being?"

Harris, who has been based in the UK for 60 years, denies 12 counts of indecent assault involving four girls, aged from seven to 19, between 1968 and 1986. The trial continues tomorrow, when Harris will be cross-examined by the prosecution.

Drawings confiscated

In an unusual diversion, the court was told that jury members had spotted Rolf Harris drawing while in the dock last week, and raised their concerns. It is forbidden to make any images while in a court – court artists create their sketches elsewhere from written notes – and particularly to portray jurors, who remain anonymous. Without saying what the sketches showed, the judge, Mr Justice Sweeney, assured the jurors that they had been confiscated and destroyed. They should not, he said, draw any adverse inferences from Harris's actions.